usb: dwc3: gadget: purge dwc3_stop_active_transfers()

That function is unnecessarily called from
dwc3_gadget_reset_interrupt(). Gadget drivers (and
thus, functions) are required to dequeue all pending
requests when they get notified about a USB Bus
Reset.

Trying to make sure there are no pending requests
only serves the purpose of working around possibly
bad gadgets.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>